JP6678669B2 - Double-layer three-way catalyst with improved aging stability - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a three-way catalyst which is composed of catalyst active layers arranged one above another and is suitable for purifying exhaust gas from a combustion engine.

  Three-way catalysts are used for purifying exhaust gases from combustion engines that are operated essentially stoichiometrically. In stoichiometric operation, the amount of air supplied to the engine exactly corresponds to the amount required for complete combustion of the fuel. In this case, the air-fuel ratio λ (also called the air ratio) is exactly 1. At around λ = 1, the three-way catalyst can simultaneously convert hydrocarbons, carbon monoxide, and nitrogen oxides into harmless compounds.

  In general, platinum group metals are used as catalytically active materials, in particular platinum, palladium and rhodium, for example, are present on λ-type aluminum oxide as carrier material. In addition, the three-way catalyst includes an oxygen storage material, for example, a cerium / zirconium mixed oxide. In the latter case, cerium oxide (cerium oxide) is a rare earth metal oxide and constitutes a component essential for oxygen storage. For zirconium oxide and cerium oxide, these materials may include additional components such as rare earth metal oxides or alkaline earth oxides. The oxygen storage material is activated by applying a catalytically active material, such as a platinum group metal, and thus also serves as a support material for the platinum group metal.

  The components of the three-way catalyst may be present in a single coating layer on the inert catalyst support, see, for example, EP 1 154 220 (A1).

  However, it is often the case that bilayer catalysts are used, which facilitate the separation of different catalytic processes and thus allow for an optimal adjustment of the catalytic effect in the two layers. Catalysts of the latter type are described, for example, in WO 95/35152 (A1), WO 2008/000449 (A2), EP 0 885 650 (A2), EP 1046423 (A2) and EP 1 726 359. No. (A1) and No. 1974809 (A1).

  EP 1974809 (A1) discloses double-layer three-way catalysts containing a cerium / zirconium mixed oxide in both layers, wherein the cerium / zirconium mixed oxide is in the upper layer and in the lower layer. Each has a higher proportion of zirconium.

  EP 1900146 (A2) discloses double layer three-way catalysts comprising mixed oxides of cerium, zirconium and niobium in both layers and, in addition, CeZrYLa aluminum oxide particles in the lower layer. Is disclosed.

  EP 1 726 359 (A1) discloses double-layer three-way catalysts containing a cerium / zirconium / lanthanum / neodymium mixed oxide having a zirconium content of more than 80 mol% in both layers. The mixed oxide / zirconium / lanthanum / neodymium oxide may each have a higher proportion of zirconium in the upper layer than in the lower layer.

  WO 2008/000449 (A2) also discloses a double layer three-way catalyst comprising a cerium / zirconium mixed oxide in both layers, wherein the mixed oxide in the upper layer also has a higher proportion of zirconium (Plural) are disclosed. The cerium / zirconium mixed oxide can to some extent be replaced by a cerium / zirconium / lanthanum / neodymium mixed oxide or a cerium / zirconium / lanthanum / yttrium mixed oxide.

  WO 2009/012348 (A1) further discloses three-way catalysts in which only the middle and upper layers contain an oxygen storage material.

  The demand for reduced emissions from combustion engines is constantly increasing, necessitating further continuous development of catalysts. In Europe, the endurance requirement has been raised to 160,000 km with emission regulations Euro 5. In the United States, further, durability requirements are up to 150,000 miles. Therefore, the aging stability of the catalyst is even more important. The key indicator of activity after aging is, on the one hand, the starting temperature of pollutant conversion by the catalyst, and, on the other hand, its dynamic conversion capacity. The starting temperature for a contaminant refers to the temperature at which the contaminant is being converted, for example, by more than 50%. The lower this temperature, the sooner the contaminants can be converted after a cold start. At full load, exhaust gas temperatures up to 1,050 ° C. may occur directly at the motor output. The better the temperature stability of the catalyst, the closer it can be placed to the engine. This also improves exhaust gas purification after a cold start.

  When the emission regulations Euro 6c came into force in September 2017, European emission regulations stipulate emissions measurement under actual operating conditions. Depending on the operating conditions, this may mean that the catalyst has much more stringent requirements, especially with regard to the dynamic conversion of carbon monoxide and nitrogen oxides. The catalyst must still meet these stringent requirements even after aging of strength. Also for this reason, it is necessary to further enhance the aging stability of the three-way catalyst.

  The above-mentioned prior art catalysts have very good properties with regard to the starting temperature and the dynamic conversion capacity after aging. However, the stricter legal requirements require that even better catalysts be considered.

  For this reason, it was an object of the present invention to provide a catalyst which, due to its higher temperature stability, has an even lower starting temperature and an improved post-aging dynamic conversion capacity compared to prior art catalysts. Was.

  Surprisingly, this problem can be solved if the rare earth element present as a component of the oxygen storage material, and optionally the platinum group metal, is distributed in a particular way on the two layers of the double-layer three-way catalyst. Was found.

The object of the present invention is therefore a catalyst comprising two layers on an inert catalyst support,
Layer A comprises at least one platinum group metal, and a cerium / zirconium / SE mixed oxide;
The layer B applied to the layer A comprises at least one platinum group metal, as well as a cerium / zirconium / SE mixed oxide;
(Where SE represents a rare earth metal other than cerium)
The percentage of SE oxide in the cerium / zirconium / SE mixed oxide of layer A is calculated as the percentage by weight in cerium / zirconium / SE mixed oxide in layer B, respectively. Is smaller than the ratio of SE oxides.

  Layer A and layer B comprise, independently of one another, as platinum group metal, in particular platinum, palladium, rhodium or a mixture of at least two of these platinum group metals.

  In some embodiments of the present invention, layer A includes platinum, palladium, or platinum and palladium, and layer B includes palladium, rhodium, or palladium and rhodium.

  In a further embodiment of the invention, the catalyst according to the invention does not contain platinum.

  In particular, layer A contains palladium and layer B contains rhodium or palladium and rhodium.

  The cerium / zirconium / SE mixed oxide can serve as a support material for the platinum group metal in layer A and / or layer B. Furthermore, in the layer A and / or the layer B, all or a part thereof can use activated aluminum oxide as a carrier.

Therefore, in embodiments of the present invention, layer A and layer B include active aluminum oxide. It is particularly preferred that the active aluminum oxide is stabilized, in particular by doping with lanthanum oxide. Preferred active aluminum oxide, containing from 1 to 6 wt.%, Particularly 3-4% by weight of lanthanum oxide _(La 2 _{O 3).}

The term "activated aluminum oxide" is known to those skilled in the art. In particular, it refers to λ-type aluminum oxide with an active aluminum oxide surface of 100 to 200 m ² / g, which has been described many times in the literature and is commercially available.

  The term “cerium / zirconium / SE mixed oxide” excludes, within the meaning of the invention, a physical mixture of cerium oxide, zirconium oxide and SE oxide. In fact, "cerium / zirconium / SE mixed oxides" are characterized by a substantially uniform, three-dimensional crystal structure, ideally free of phases from cerium oxide, zirconium oxide or SE oxides. However, depending on the production process, a completely homogeneous product can be obtained and can generally be used without any inconvenience.

  For example, lanthanum oxide, yttrium oxide, praseodymium oxide, neodymium oxide, samarium oxide, and mixtures of one or more of these metal oxides are considered as rare earth metal oxides in cerium / zirconium / SE mixed oxides. You may.

  Preferred are lanthanum oxide, yttrium oxide, praseodymium oxide, and mixtures of one or more of these metal oxides. Particularly preferred are lanthanum oxide and yttrium oxide, with mixtures of lanthanum oxide and yttrium oxide being particularly preferred.

  According to the invention, the percentage of SE oxide in the cerium / zirconium / SE mixed oxide of layer A is calculated as the percentage by weight of cerium / zirconium / SE mixed oxide in layer B, respectively, / SE is smaller than the ratio of the SE oxide in the mixed oxide.

  The proportion of SE oxide in layer A is in particular 1 to 12% by weight, preferably 3 to 10% by weight, still more preferably 6 to 9% by weight, based on the cerium / zirconium / SE mixed oxide in each case. %.

  The proportion of SE oxide in layer B is, in particular, from 2 to 25% by weight, preferably from 10 to 20% by weight, even more preferably from 14 to 18% by weight, based on the cerium / zirconium / SE mixed oxide in each case. %.

  According to the present invention, the ratio of cerium oxide to zirconium oxide in the cerium / zirconium / SE mixed oxide can vary widely. In the layer A, the ratio is, for example, 0.1 to 1.0, preferably 0.2 to 0.7, and still more preferably 0.3 to 0.5.

  In the layer B, the ratio is, for example, 0.1 to 1.0, preferably 0.2 to 0.7, and still more preferably 0.3 to 0.5.

  The cerium / zirconium / SE mixed oxide in the present invention does not particularly contain aluminum oxide.

  In embodiments of the present invention, one or both layers include an alkaline earth compound such as barium oxide or barium sulfate. A preferred embodiment includes barium sulfate in layer A. The amount of barium sulfate is in particular from 5 to 20 g / L, based on the volume of the inert catalyst support.

  In a further embodiment of the invention, one or both layers additionally comprise additives such as rare earth compounds, for example lanthanum oxide, and / or binders, for example aluminum compounds. These additives are used in widely variable amounts and can be easily determined by the skilled person in a particular case.

One embodiment of the present invention is a catalyst comprising two layers on an inert catalyst support,
Layer A comprises palladium, activated aluminum oxide, and a cerium / zirconium / lanthanum / yttrium mixed oxide;
Layer B applied to layer A comprises rhodium or palladium and rhodium, activated aluminum oxide, and a mixed oxide of cerium / zirconium / lanthanum / yttrium;
The total percentage of lanthanum oxide and yttrium oxide in the cerium / zirconium / lanthanum / yttrium mixed oxide of layer A is calculated as the weight percentage of cerium / zirconium / lanthanum / yttrium oxide in each case. The present invention relates to a catalyst characterized by being smaller than the total ratio of lanthanum oxide and yttrium oxide in a zirconium / lanthanum / yttrium mixed oxide.

  In this case, the total proportion of lanthanum oxide and yttrium oxide, calculated in each case as a percentage by weight relative to the cerium / zirconium / lanthanum / yttrium mixed oxide, is calculated as 6 to 9% by weight based on the cerium / zirconium / lanthanum / yttrium mixed oxide of the layer A, and in the cerium / zirconium / lanthanum / yttrium mixed oxide of the layer B, The content is preferably 14 to 18% by weight based on the mixed oxide.

  In a further embodiment of the invention, layer A is provided directly on the inert catalyst support, ie there are no additional layers or undercoat between the inert catalyst support and layer A.

  In a further embodiment of the invention, layer B is in direct contact with the exhaust gas stream, ie there are no additional layers or overcoats on layer B.

  In a further embodiment of the invention, the catalyst according to the invention is composed of layers A and B on an inert catalyst support. This means that layer A is provided directly on the inert catalyst support, layer B is in direct contact with the exhaust gas stream, and that no other layers are present.

  A honeycomb body made of ceramic or metal and having a volume V has a parallel flow path of exhaust gas of a combustion engine, and is particularly suitable as a catalyst carrier inert to a catalyst. They may be either so-called flow-through type honeycomb bodies or wall-flow type filters.

  According to the invention, the wall region of the channel is coated with two catalyst layers A and B. To coat the catalyst support with layer A, the solids provided in this layer are suspended in water and coated using the coating suspension thus obtained of the catalyst support. This process is repeated with the coating suspension and the solids provided for Layer B are suspended in water. Preferably, both layers A and B are coated along the entire length of the inert catalyst support. This means that layer B completely covers layer A, so that only layer B comes into direct contact with the exhaust gas stream.

In the following Examples 1 to 3 and Comparative Example 1, the double-layer catalyst was made from a ceramic having a cell number of 93 / cm ² and a wall thickness of 0.09 mm, and dimensions of 11.8 cm in diameter and 10.5 cm in length. It was manufactured by coating the flow-through type honeycomb body twice. For this purpose, two different suspensions were prepared for layer A and for layer B, respectively. Next, the carrier was first coated with the suspension for Layer A and then calcined at 500 ° C. in air for 4 hours. Thereafter, the carrier coated with layer A was coated with the suspension for layer B and then calcined under the same conditions as for layer A.

Example 1
A double layer catalyst was prepared by first preparing two suspensions. The composition of the first suspension for layer A was (relative to the volume of the catalyst support) as follows:
40 g / L of activated aluminum oxide stabilized with 4% by weight of La ₂ O ₃ ,
40 g / L cerium / zirconium / lanthanum / yttrium mixed oxide (25 wt% CeO ₂ , 67.5 wt% ZrO ₂ , 3.5 wt% La ₂ O ₃ , and 4 wt% Y ₂ O ₃ ),
5 g / L BaSO ₄ ,
3.178 g / L Pd.
The composition of the second suspension for layer B was (relative to the volume of the catalyst support) as follows:
60 g / L of activated aluminum oxide stabilized with 4% by weight of La ₂ O ₃ ,
47 g / L cerium / zirconium / lanthanum / yttrium mixed oxide (24 wt% CeO ₂ , 60 wt% ZrO ₂ , 3.5 wt% La ₂ O ₃ , and 12.5 wt% Y ₂ O ₃ ),
0.177 g / L Pd,
0.177 g / L Rh.

Example 2
A double-layer catalyst was produced in the same manner as in Example 1. The composition of the first suspension for Layer A was as follows.
40 g / L of activated aluminum oxide stabilized with 4% by weight of La ₂ O ₃ ,
40 g / L cerium / zirconium / lanthanum / yttrium mixed oxide (20.5 wt% CeO ₂ , 67.5 wt% ZrO ₂ , 4.5 wt% La ₂ O ₃ , and 7.5 wt% Y ₂ O ₃ ),
5 g / L BaSO ₄ ,
3.178 g / L Pd.
The composition of the second suspension for Layer B was as follows.
60 g / L of activated aluminum oxide stabilized with 4% by weight of La ₂ O ₃ ,
47 g / L of cerium / zirconium / lanthanum / yttrium mixed oxide (20 wt% of _CeO 2, 60 wt% of _ZrO 2, 5 wt% _La 2 _{O 3,} and 15 wt% of _Y 2 _O 3),
0.177 g / L Pd,
0.177 g / L Rh.

Example 3
A double-layer catalyst was produced in the same manner as in Example 1. The composition of the first suspension for Layer A was as follows.
40 g / L of activated aluminum oxide stabilized with 4% by weight of La ₂ O ₃ ,
40 g / L cerium / zirconium / lanthanum / yttrium mixed oxide (20.5 wt% CeO ₂ , 67.5 wt% ZrO ₂ , 4.5 wt% La ₂ O ₃ , and 7.5 wt% Y ₂ O ₃ ),
5 g / L BaSO ₄ ,
3.178 g / L Pd.
The composition of the second suspension for Layer B was as follows.
60 g / L of activated aluminum oxide stabilized with 4% by weight of La ₂ O ₃ ,
47 g / L of cerium / zirconium / lanthanum / yttrium mixed oxide (15 wt% of _CeO 2, 60 wt% of _ZrO 2, 7 wt% _La 2 _{O 3,} and 18 wt% of _Y 2 _O 3),
0.177 g / L Pd,
0.177 g / L Rh.

Comparative Example 1
A double-layer catalyst was produced in the same manner as in Example 1. The composition of the first suspension for Layer A was as follows.
40 g / L of activated aluminum oxide stabilized with 4% by weight of La ₂ O ₃ ,
40 g / L cerium / zirconium / lanthanum / yttrium mixed oxide (25 wt% CeO ₂ , 67.5 wt% ZrO ₂ , 3.5 wt% La ₂ O ₃ , and 4 wt% Y ₂ O ₃ ),
5 g / L BaSO ₄ ,
3.178 g / L Pd.
The composition of the second suspension for Layer B was as follows.
60 g / L of activated aluminum oxide stabilized with 4% by weight of La ₂ O ₃ ,
47 g / L cerium / zirconium / lanthanum / yttrium mixed oxide (25 wt% CeO ₂ , 67.5 wt% ZrO ₂ , 3.5 wt% La ₂ O ₃ , and 4 wt% Y ₂ O ₃ ),
0.177 g / L Pd,
0.177 g / L Rh.

  Example 1 and Comparative Example 1 were aged in an aging process on an engine (engine) test bench. The aging process consists of aging due to inertia fuel cutoff at 950 ° C exhaust gas temperature in front of the catalyst inlet (bed temperature up to 1,030 ° C). The aging deterioration time was set to 76 hours.

  The starting performance was tested on an engine test bench at an average air-fuel ratio of λ constant, and the dynamic conversion was tested by changing λ.

Table 1 describes the temperature T _50, which represents 50% of the components discussed are converted. In this way, the starting performance based on the stoichiometric exhaust gas composition (λ = 0.999, ± 3.4% swing) was determined.

  The dynamic conversion performance was determined at a constant temperature of 510 ° C. with λ in the range of 0.99 to 1.01. At this time, the swing width of λ was ± 3.4%. Table 2 lists the conversions at the intersection of the CO and NOx conversion curves, and the associated HC conversions.

  Example 1 according to the present invention shows a remarkable improvement in the starting performance and the dynamic conversion of CO / NOx after aging.

In Examples 4 and 5 below and Comparative Example 2, the double-layer catalyst was made from a ceramic having a cell number of 93 / cm ² and a wall thickness of 0.1 mm, and dimensions of 10.2 cm in diameter and 15.2 cm in length. It was manufactured by coating the flow-through type honeycomb body twice. For this purpose, two different suspensions were prepared for layer A and for layer B, respectively. Next, the carrier was first coated with the suspension for Layer A and then calcined at 500 ° C. in air for 4 hours. Thereafter, the carrier coated with layer A was coated with the suspension for layer B and then calcined under the same conditions as for layer A.

Example 4
A double layer catalyst was prepared by first preparing two suspensions. The composition of the first suspension for layer A was (relative to the volume of the catalyst support) as follows:
70 g / L of activated aluminum oxide stabilized with 4% by weight of La ₂ O ₃ ,
50 g / L of cerium / zirconium / lanthanum / yttrium mixed oxide (39 wt% _CeO 2, 51 wt% of _ZrO 2, 3 wt% _La 2 _{O 3,} and 7 wt% of _Y 2 _O 3),
5 g / L BaSO ₄ ,
1.483 g / L Pd.
The composition of the second suspension for layer B was (relative to the volume of the catalyst support) as follows:
70 g / L of activated aluminum oxide stabilized with 4% by weight of La ₂ O ₃ ,
65 g / L cerium / zirconium / lanthanum / yttrium mixed oxide (24 wt% CeO ₂ , 60 wt% ZrO ₂ , 3.5 wt% La ₂ O ₃ , and 12.5 wt% Y ₂ O ₃ ),
0.177 g / L Pd,
0.177 g / L Rh.

Example 5
A double-layer catalyst was produced in the same manner as in Example 4. The composition of the first suspension for Layer A was as follows.
70 g / L of activated aluminum oxide stabilized with 4% by weight of La ₂ O ₃ ,
50 g / L cerium / zirconium / lanthanum / yttrium mixed oxide (25 wt% CeO ₂ , 67.5 wt% ZrO ₂ , 3.5 wt% La ₂ O ₃ , and 4 wt% Y ₂ O ₃ ),
5 g / L BaSO ₄ ,
1.483 g / L Pd.
The composition of the second suspension for Layer B was as follows.
70 g / L of activated aluminum oxide stabilized with 4% by weight of La ₂ O ₃ ,
65 g / L cerium / zirconium / lanthanum / yttrium mixed oxide (24 wt% CeO ₂ , 60 wt% ZrO ₂ , 3.5 wt% La ₂ O ₃ , and 12.5 wt% Y ₂ O ₃ ),
0.177 g / L Pd,
0.177 g / L Rh.

Comparative Example 2
A double-layer catalyst was produced in the same manner as in Example 4. The composition of the first suspension for Layer A was as follows.
70 g / L of activated aluminum oxide stabilized with 4% by weight of La ₂ O ₃ ,
50 g / L of cerium / zirconium / lanthanum / yttrium mixed oxide (39 wt% _CeO 2, 51 wt% of _ZrO 2, 3 wt% _La 2 _{O 3,} and 7 wt% of _Y 2 _O 3),
5 g / L BaSO ₄ ,
1.483 g / L Pd.
The composition of the second suspension for Layer B was as follows.
70 g / L of activated aluminum oxide stabilized with 4% by weight of La ₂ O ₃ ,
65 g / L of cerium / zirconium / lanthanum / yttrium mixed oxide (22 wt% _CeO 2, 68 wt% of _ZrO 2, 2 weight% of _La 2 _O 3, 5 wt% of _Nd 2 _{O 3,} and 3 weight % Y ₂ O ₃ ),
0.177 g / L Pd,
0.177 g / L Rh.

  Examples 4 and 5 and Comparative Example 2 were aged over time in an aged deterioration process on an engine test bench. The aging process consists of aging due to inertia fuel cutoff at 950 ° C exhaust gas temperature in front of the catalyst inlet (bed temperature up to 1,030 ° C). The aging deterioration time was set to 76 hours.

  The starting performance was tested on an engine test bench at an average air-fuel ratio of λ constant, and the dynamic conversion was tested by changing λ.

  Table 3 lists the temperature T50 at which 50% of each of the components studied was converted. As such, due to stoichiometric exhaust gas composition (λ = 0.999, ± 3.4% swing) and slightly lean exhaust gas composition (λ = 1.05, no swing) The starting performance was determined.

  The dynamic conversion performance was determined at a constant temperature of 510 ° C. with λ in the range of 0.99 to 1.01. At this time, the swing width of λ was ± 3.4%. Table 4 lists the conversions at the intersection of the CO and NOx conversion curves, and the associated HC conversions.

  Examples 4 and 5 according to the invention show a marked improvement in the starting performance and in the dynamic conversion of CO / NOx after aging, with Example 5 showing a particularly high activity.

A further embodiment, similarly to Embodiment 5, the difference that certain rare earth metal oxides (SE x _{O y)} used in the cerium / zirconium / rare earth metal mixed oxides in Table 5 were prepared.

Claims (13)

A three-way catalyst comprising two layers on an inert catalyst support,
Layer A comprises at least one platinum group metal, and a cerium / zirconium / SE mixed oxide;
The layer B applied to the layer A comprises at least one platinum group metal, as well as a cerium / zirconium / SE mixed oxide;
(Where SE represents a rare earth metal other than cerium)
The percentage of SE oxide in the cerium / zirconium / SE mixed oxide of layer A is calculated as the percentage by weight of the cerium / zirconium / SE mixed oxide in layer A, respectively. rather smaller than the proportion of the SE oxide in the oxide,
Layer A and layer B comprise active aluminum oxide;
The SE oxide in the cerium / zirconium / SE mixed oxide in at least one of Layer A and Layer B is lanthanum oxide, yttrium oxide, praseodymium oxide, samarium oxide, or one of these metal oxides A catalyst , which is a mixture of the above .   2. The catalyst according to claim 1, wherein the layers A and B comprise, independently of one another, as platinum group metal platinum, palladium, rhodium or a mixture of at least two of these platinum group metals. . As the platinum group metals, the layer A is, platinum, palladium, or comprises a platinum and palladium, the layer B, palladium, rhodium, or, characterized in that it comprises a palladium and rhodium, according to claim 1 or 2 Catalyst.   The catalyst according to any one of claims 1 to 3, wherein the layer A contains palladium as the platinum group metal, and the layer B contains rhodium or palladium and rhodium. The catalyst according to any one of claims 1 to 4, wherein all or a part of the platinum group metal in the layer A and / or the layer B uses activated aluminum oxide as a carrier. The SE oxide of the cerium / zirconium / SE mixed oxide in at least one layer A and layer B, characterized in that a mixture of lanthanum oxide yttrium oxide, any of claim 1-5 The catalyst according to claim 1. The ratio of the SE oxide in the cerium / zirconium / SE mixed oxide in the layer A is 1 to 12 % by weight based on the cerium / zirconium / SE mixed oxide. The catalyst according to any one of claims 1 to 6 . Ratio of the SE oxide of the cerium / zirconium / SE mixed oxide in the layer B is, over the previous SL cerium / zirconium / SE mixed oxide, characterized in that it is a 2 to 25% by weight, wherein Item 8. The catalyst according to any one of Items 1 to 7 . The weight ratio of cerium oxide to zirconium oxide in the cerium / zirconium / SE mixed oxide in layer A is 0.1 to 1. Characterized in that it is a 0, the catalyst according to any one of claims 1-8. The weight ratio of cerium oxide to zirconium oxide in the cerium / zirconium / SE mixed oxide in layer B is 0.1 to 1. Characterized in that it is a 0, the catalyst according to any one of claims 1-9. The catalyst according to any one of claims 1 to 10 , comprising two layers on an inert catalyst support,
Layer A comprises palladium, activated aluminum oxide, and a cerium / zirconium / lanthanum / yttrium mixed oxide;
Layer B applied to Layer A comprises rhodium, or palladium and rhodium, activated aluminum oxide, and cerium / zirconium / lanthanum / yttrium mixed oxide;
The total percentage of lanthanum oxide and yttrium oxide in the cerium / zirconium / lanthanum / yttrium mixed oxide of layer A is calculated as the percentage by weight based on cerium / zirconium / lanthanum / yttrium oxide, respectively. A catalyst characterized by being smaller than the total ratio of lanthanum oxide and yttrium oxide in a cerium / zirconium / lanthanum / yttrium mixed oxide. The total proportion of lanthanum oxide and yttrium oxide, calculated in each case as a percentage by weight relative to the cerium / zirconium / lanthanum / yttrium mixed oxide, in the cerium / zirconium / lanthanum / yttrium mixed oxide of layer A: 6 to 9% by weight based on the cerium / zirconium / lanthanum / yttrium mixed oxide of the layer A, and the cerium / zirconium / lanthanum of the layer B in the cerium / zirconium / lanthanum / yttrium mixed oxide of the layer B. The catalyst according to claim 11 , characterized in that the content is 14 to 18% by weight based on the mixed oxide of yttrium and yttrium. The catalyst according to any one of claims 1 to 12 , wherein the layer A is provided directly on the inert catalyst support.